After Yuuto gets hurt protecting her, Himari has an Heroic BSOD, flips into an Unstoppable Rage, and dismembers a giant by slicing its right arm off. She then proceeds to nearly kill Ageha, another demon who attacked them, and is only stopped by Yuuto, who's afraid her Superpowered Evil Side might take over.
The first time Yuuto steps in to break up a fight with the fact that he really believes in his heart that everyone can get along as his only weapon.
In the climax of the anime, Himari is forced to go fully over to her Superpowered Evil Side (which includes a further physical transformation). Yuuto manages to stop her from killing him with a hug. Yes, a hug. He then goes on to fully empower her to kill Shuten-douji. This in the anime marks his first conscious use of his Demon Slayer power, and may also count as Took a Level in Badass.